Deadly Strike gives you an X% chance of dealing double Damage. The damage is doubled after all other bonuses, including Strength and Dexterity modifiers. Deadly Strike will not stack with the Assassin's, Barbarian's or Amazon's Critical Strike bonuses, although there are advantages to having both. When you have both Deadly Strike and Critical Strike, the game will roll for one of the bonuses first and if that fails, the game will then roll for the other bonus. If the first bonus succeeds, however, there is no roll for the second bonus as the game only allows for 2X damage, never 4X damage. That means that the chance to hit will be CS + (CS * (1 - DS)); so if you had 25% Critical Strike and 25% Deadly Strike, the chance of dealing double damage would be: .25 + (.25 * .75) = 43.75%.